Free fatty acids(FFAs)are known to play important roles in the quality and authenticity of edible oils,but determination of FFAs is complicated challenging.The existing difficulties for the analysis of FFAs in edible oils include the trace amount of FFAs in oils,difficult to be purified and enriched from complicated oil matrix,and also the poor ionization efficiency and sensitivity deficiency when determining FFAs under the negative ion mode of mass spectrometry.Herein,a strategy based on simple chemical derivatization coupled with neutral loss scan-shotgun-electrospray ionization-tandem quadrupole mass spectrometry(NLS-ESI-MS/MS)for analysis of FFAs in edible oils was developed.A derivative reagent(N,N-diethyl-1,2-ethanediamine,DEEA)was employed to selectively label carboxyl groups of FFAs to form an amino compound with a tertiary amino group,which enabled to analyze of DEEA derived FFAs under the positive ion mode of mass spectrometry.The DEEA derivative products can lose a characteristic neutral loss fragment of 73 Da(NH(CH3CH2)2)in collision-induced dissociation,which enabled us to discriminate and analyze of DEEA derived FFAs with neutral loss scan mode(NLS73Da).After DEEA derivatization,the detection sensitivities of FFAs improved by 1700-6670 folds compared with underived FFAs detected with MS in negative ion mode.The assay was linear over the concentration range 0.5-200nmol/L with satisfactory correlation coefficients(R2>0.9942),whilst the limit of detection and quantitation were 0.1-0.5nmol/L and 0.3-2.0 nmol/L,respectively.Finally,the established method was applied to determine dynamic FFA formation in eight types of edible oils subjected to an oil-bath heating treatment test.By using this method,oil samples can be simply diluted by n-hexane,derived with DEEA,and then directly infused and analyzing the derivatives by NLS-ESI-MS/MS without complicated sample purification treatment.The results showed that 11 FFAs could be distinctly detected and the contents of C18:1,C18:2 and C18:3 in all investigated oil samples and the contents of C20:4,C20:5 and C22:6 in fish oil sample had significant changes,demonstrating that these FFAs may associate with thermal degradation process of edible oils.The developed method was useful for the rapid and sensitive detection of FFAs in edible oil samples and offers unique advantages and presents a powerful tool for the research of thermal degradation mechanism of edible oils.